A circuit arrangement is disclosed in commonly assigned published patent Publication 0 123 243 A2 of the European Patent Office (corresponding to U.S. Pat. No. 4,538,269) in which it is possible to inject a test signal from outside via a signal input and the normal signal path. This test signal is then taken off directly behind the signal input and can be reinjected into the signal path at another point via selecting multiplexers, with a portion of the signal path being bypassed. If such an arrangement is to be tested at regular intervals during operation, it must be either regularly removed and connected to a measuring setup, as is customary in prior art telephone switching systems, or connected to a measuring setup via existing signal paths. On the one hand, this necessitates two additional connections from and to the measuring setup, which represent an additional traffic load, and on the other hand, errors on such additional paths enter fully into the error measurement. Furthermore, only few tests can be made simultaneously and long-term measurements are therefore virtually impossible.
From DE-OS 33 31 403 it is known to build complete self-test circuitry into electronic equipment which tests the equipment on request and makes the results of the test accessible. In principle, it is possible, e.g., in a telephone exchange, to equip all or a great number of assemblies with such self-test circuitry. In the case of large-scale-integrated assemblies, the additional expense would probably not be altogether prohibitive. The necessary exchange of information with the environment would also be acceptable, because it would only be necessary to initiate the test and later interrogate the testing device for the result. For tests covering two or more assemblies, however, a corresponding number of such self-test circuits are necessary.